Centrifugal compressor diffuser

ABSTRACT

A diffuser for a centrifugal compressor is a modification of those in which passages of circular cross section merge at the entrances to the passage to define generally elliptical leading edges of the walls which separate the passages. The entrance portion of each passage is defined by a basic cross section based upon centerlines which converge in the direction of flow and merge to define a single centerline in the downstream portion of the passages. The basic cross section of the passage is defined by an arc approximating a semicircle centered on each branch of the centerline and straight or substantially straight lines connecting the ends of the arcs. The upstream centerline in the direction of gas flow of each passage intersects the downstream centerline of the next forward passage at the inner periphery of the diffuser. The resulting intersection of the boundaries of the diffusing passages results in a generally elliptical leading edge of the walls which lie between the passages.

United States Patent [191 Fitzpatrick [451 Sept. 16, 1975 CENTRIFUGAL COMPRESSOR DIFFUSER [75] Inventor: Paul Fitzpatrick, Detroit, Mich.

[73] Assignee: General Motors Corporation,

Detroit, Mich.

[22] Filed: Sept. 3, 1974 [21] Appl. No.: 502,830

[52] U.S. Cl. 415/211; 415/181; 415/207 3,778,186 12/1973 Bandukwalla 415/211 3,860,360 1/1975 Yu 415/211 FOREIGN PATENTS OR APPLICATIONS 419,544 1l/1934 United Kingdom 415/211 Primary Examiner-I-Ienry F. Raduazo Attorney, Agent, or Firm-J. C. Evans [5 7] ABSTRACT A diffuser for a centrifugal compressor is a modification of those in which passages of circular cross section merge at the entrances to the passage to define generally elliptical leading edges of the walls which separate the passages. The entrance portion of each passage is defined by a basic cross section based upon centerlines which converge in the direction of flow and merge to define a single centerline in the downstream portion of the passages. The basic cross section of the passage is defined by an arc approximating a semicircle centered on each branch of the centerline and straight or substantially straight lines connecting the ends of the arcs. The upstream centerline in the direction of gas flow of each passage intersects the downstream centerline of the next forward passage at the inner periphery of the diffuser. The resulting intersection of the boundaries of the diffusing passages results in a generally elliptical leading edge of the walls which lie between the passages.

5 Claims, 7 Drawing Figures PATENTEB SEP I 8 I975 sum 2 u; 2

C ENTRIFUGAL COMPRESSOR DIFFUSER quirements of practice and providing for greater latitude in the constraintsimposed upon the designer of a diffuser for any specific application.

Centrifugal compressors for air and other gases are very well known and widely employed and have great advantages, particularly in lower pressureranges, because of the simplicity of their structure as compared to axial-flow compressors.

Such a compressor is described in Atkinson US. Pat. No. 2,819,012 for Centrifugal Compressor, Jan. 7, 1958. Air enters the compressor through an annular entrance generally parallel to the axis of rotation and is accelerated circumferentially of the rotor while moving radially outward under the influence of centrifugal force, the discharge of air from the impeller being primarily tangential, with a substantial radial component and ordinarily little axial component of velocity.

The high velocity discharge from the rotor or impeller is diffused in structure providing a diverging flow path and the dynamic head is thus in large measure converted to pressure head, so that the discharge is at a substantial rise in pressure above atmospheric or other inlet pressure. Such compressors are found in gas turbine engines, turbochargers, air conditioning machines, and various other environments.

In. the more usual case each diffusing passage diverges from its inlet through to its outlet; however, where the gas upon entry to the diffuser is moving at supersonic speed, it may be desirable to have the diffusing passages converge initially to a throat and then diverge. An example of this is described in Bandukwalla U.S. Pat. No. 3,778,136, Dec. 11, 1973. In some applications of centrifugal compressors, conditions of flow vary quite considerably as, for example, with power level of a gas turbine engine in which such a compressor is employed. One effect which is significant in the design of diffusers is the variation in direction of flow from the rotor with such changes of operating condition. By direction of-flow is meant the angle which the gas flow makes to the radial or circumferential direction at the point of entry to the diffuser. It may also be regarded as the relation between the radial and circumferential components of the gas velocity.

One approach to design of diffusers has resulted in a series of disclosures of what have come to be known as pipe diffusers. Such are found in US. Pat. Nos. as follows: Vrana No. 3,333,762, August 1, 1967; Jarosz et al No. 3,420,435, Jan. 7, 1969; Cronstedt No. 3,604,818, Sept. '14, 1971; OConnor Pat. No. 3,706,510, Dec. 19, 1972; and O'Connor No. 3,743,436, July 3, 1973..These show diffusers lacking the usual rectangular entrances to the diffusing passages determined by the leading edges of vanes which extend parallel or approximately parallel to the axis of rotation of the compressor rotor. Instead, the entrance to each diffusing passage is bounded by a generally elliptical or V-shaped leading edge as determined by the intersection between passages which depart from the usual rectangular form bounded by lines parallel and perpendicular to the axis ,of rotation of the rotor.

It has been asserted that this configuration provides a compressor which is more tolerant of variations of flow through the compressor than those with the usual flat plate of vane boundary between the diffusing passages, with high pressure rise capability and good efficiency. It is apparent that following the teaching of such disclosures places rather heavy constraints upon the designer of a centrifugal compressor. For one item, the centerlines of the passages are tangent to a circle very closely adjacent to the circumference of the compressor rotor.

For the detailed design of a centrifugal compressor it is well recognized that many things must be taken into consideration, such as volume of air flow, pressure ratio, and speed of rotation of the rotor. In many cases, it may be desirable to have the entrances to the diffusing passages make a higher angle with the circumference of the rotor than is possible when tney are tangent to a circle near the circumference of the rotor. It also may be desirable in some installations to have the outlets of the diffusing passages approach a radial direction. This may be particularly the case where the compressed air is delivered to a regenerator in a gas turbine engine such as those described in the US. Pat. Nos. to Collman et al. No. 3,077,074, Feb. 12, 1963; Collman et al. No. 3,267,674, Aug. 23, 1966; and Bell No. 3,680,983, Aug. 1, 1972.

It has occurred to me that much greater flexibility of design of the compressor diffuser may be had by adopting a basic cross section of the diffusing passages which changes from what may be termed a racetrack curve at the entrance to a circle at an intermediate point of the diffuser. This racetrack curve if exactly or approximately the curve defined by two approximately semicircular arcs of the same or similar radius and by straight lines connecting the ends of the arcs, thus defining a generally oval shape. With this configuration, the intersection between the passages defining the generally elliptical leading edge of the wall which separates the passages may be obtained with curved passages and with passages which leave the rotor at a considerable outward angle rather than substantially tangentially. Further, as is pointed out in the copending application of Yu Ser. No. 394,356 filed Sept. 4, 1973, now US. Pat. No. 3,860,360, of common ownership, it is desirable to have the hydraulic radius of the diffusing passages at a maximum; in other words, to have the passages approach a circular configuration as opposed, for example, to a rectangular one, in which the ratio of the bounding circumference of the passage to its area is greater than in the case of a circular passage. The diffuser configuration to be described herein facilitates approach to the desired circular cross section and transitions from a noncircular racetrack configuration of relatively high hydraulic radius to the preferred circular configuration in the direction of flow through the diffusing passage.

The principal objects of my invention are to provide improved centrifugal compressors; to provide centrifugal compressors having high flexibility as to conditions of operation at higher efficiency over a wider range of pressure conditions; to provide a diffuser tolerant of changes in flow conditions; to provide a diffuser in which diffusing passages are separated by walls having approximately elliptical leading edges and in which the direction of departure of the passages from the rotor and the curvature of the centerlines of the passages may be widely varied.

The nature of my invention and the manner in which these objects are achieved will be apparent to those skilled in the art from the succeeding description of preferred embodiments of the invention and the accompanying drawings thereof.

Referring to the drawings,

FIG. 1 is a partial cross sectional view of a centrifugal compressor taken on a plane containing the axis of rotation of the compressor rotor, as indicated in FIG. 2.

FIG. 2 is a partial cross sectional view of the same taken on a central plane of the diffuser as indicated by the line 22 in FIG. 1

FIG. 3 is a partial sectional view taken on the broken plane indicated by the line 33 in FIG. 2. I

FIGS. 4A and 4B are partial sectional views taken on the plane indicated by the line 4-4 in FIG. 2.

FIG. 5 is a view similar to FIG. 2 of an alternative form of diffuser.

FIG. 6 is a diagram illustrating progressive development of a basic cross section of a diffuser passage.

Referring first to FIG. 1, the centrifugal compressor, illustrated more or less schematically, comprises a housing 2 having a front wall 3 and a rear wall 4. A rotor or impeller 6 disposed between the walls is suitably fixed to a shaft 7. The shaft is rotatably supported in a bushing 8 mounted in a bearing support 10 suitably supported from the front wall and in a bushing 11 mounted in the rear wall 4. The front wall 3 and bearing support 10 define an annular air entrance or eye 12. The front and rear walls define between them a diffuser 14.

The rotor 6 comprises a hub or disk 15 fixed on shaft 7. The hub defines the inner and rear boundary of the gas flow path through the compressor rotor. The front wall 3 defines the outer and forward boundary of this flow path.

The air is moved by a ring of generally radially extending blades 16, the initial portions of which are curved forwardly to act as the inducer portion of the impeller and the rear or radially outer portions of which lie approximately or exactly in surfaces parallel to the axis of the shaft 7. The blades terminate at a tip 18 which may be at the same radius as the periphery of the disk 15. The structure so far described is known, and may be the same as that described at greater length in the aforementioned Atkison patent. Variation in configuration of the impeller may be made as considered desirable.

Proceeding now with a description of the first form of the diffuser which is illustrated also in FIG. 2, the front and rear walls 3 and 4 of the diffuser meet at a joining surface 19 which is preferably plane and radial to the axis of shaft 7. The walls may be joined at the outer edge of the housing at flanges 20. The front and rear walls are formed by machining or otherwise to define diffusing passages 22 which are preferably half in each of the forward and rear walls so that the passages are symmetrical with respect to the joining plane 19. The passages 22 are separated by dividing walls 23, which are portions of the walls 3 and 4 which rise to the joining surface 19. The two portions 3 and 4 of the housing may be joined by bolts at the flanges or, if desired, might be cemented or otherwise fixed together there or between the passages 22. The configuration or cross section of the dividing walls 23 is determined by the cross section, centerline configuration, and angular spacing of the passages 22, as will be explained. Passages 22 may discharge into any desired sort of space, plenum, or conduit for the reception of the compressed gas. As illustrated in FIG. 1, the passages 22 discharge into a scroll 24 defined by the walls 3 and 4. If desired, and in certain installations, the outer margin of the diffuser may be at a cylindrical surface 26 which is the radially outer boundary of the pipe diffuser and radially inner boundary of the space 27 within the scroll 24 as shown.

FIG. 2 also illustrates a portion of the outer margin of the rotor 6 including the hub 15 and blades 16. The outer periphery of the rotor is indicated at 18 and a shoulder on the rear wall overlying the periphery of the hub 15 is indicated at 28. The diffusing passages 22 at their entrance are of the same width axially of the compressor as the width axially of the compressor of the ends of blades 16. Ordinarily, the passages diverge toward the outlet in both width and depth, depth being the dimension in a plane perpendicular to the axis of rotation of the compressor.

We proceed now to a description of the configuration or contours of the diffusing passages 22 and dividing walls 23. The passages 22 are based upon a branched centerline which may be considered to be in three portions. These are a forward branch centerline 30 and a rearward branch centerline 31, both indicated by bro ken lines and both originating slightly outwardly from the rotor periphery 18. The forward centerline is so described because it is upstream in the direction of flow of the gas and is forward of line 31 with respect to direction of rotation of the rotor (clockwise as illustrated in FIG. 2 and indicated by the arrow 32). Centerlines 30 and 31 converge in the direction of flow through passages 22 and ultimately merge, at a point indicated as 34, into a common centerline 35. Note that the centerline 31 of each passage intersects the centerline 30 of the adjacent passage at a point indicated as 36. This point is spaced slightly outwardly from the periphery 18 of the rotor at the radially inward ends of the branch centerlines. The amount of spacing from the rotor may be varied, and the points 36 may be considerably farther removed from the rotor than is indicated by the proportion shown. This will depend upon whether a significant amount of vaneless diffusing space around the rotor is desired. This is a matter of choice and will depend upon the nature of the installation and preferences of the designer.

In the particular example illustrated, line 30 is a straight line which makes an angle of about 15 to a line tangent to the rotor at the location of intersection point 36. In this particular example, the line 30 if continued forward would just miss the periphery of the rotor. However, if the vaneless space is greater, the space between the forward projection of line 30 and the rotor would increase. If the outward angle of line 30 should increase, then line 30 would intersect the rotor and the continuation of the line would be tangent to a cirle of smaller diameter than the rotor. Branch centerline 31 is a curved line which is determined by the location of point 36 on line 30 and point 34 on line 30. Preferably, this line is a smooth curve approximating a circular or parabolic curve or logarithmic spiral, generally as illustrated.

The basic cross section or racetrack curve which defines the diffusing passage 22 is shown more clearly in its preferred configuration in FIGS. 3 and 4. In these figures, the vertical line 19 represents the central plane of the diffuser. Lines A-A and B-B are perpendiculars to the line 19 passing through the forward branch centerline 30 and rearward branch centerline 31, respectively. These lines are parallel to the axis of rotation of the impeller. The portion of the view between lines A-A and B-B in FIG. 3 is taken in a circular cylindrical surface having its center on the axis of the compressor. The upper and lower portions of the view in FIG. 3 are substantially normal to the centerlines 30 and 31, respectively. FIGS. 4A and.4B are taken in a flat plane extending approximately perpendicularly across the centerlines. FIGS. 3 and 4A illustrate fully one passage 22 and the relation of it to adjacent passages 22. Point 39 in FIG. 3 represents the point of complete separation of two adjacent diffusing passages. The partial overlap between adjacent passages is shown in the lower part of both FIGS. 3 and 4, the intersection of the two passages being along a line 40 which is approximately a semi-ellipse extending from abreast the intersection point 36 of the branch centerlines past the point 39.

FIGS. 3 and 4A illustrate a passage which is diverging in width as it proceeds outwardly from the rotor and in which the width of the passage is constant at a given radius from the rotor axis. Thus, the line segments CC and D-D are parallel in FIG. 3, since this is a section at constant radius. On the other hand, the line segments EE and FF in FIG. 4A diverge, defining a trapezoidal figure between the semicircles. The reason, of course, is that the point of intersection with line 30 is at a greater radius in FIG. 4 than the point of intersection with line 31. FIG. 4A more clearly represents the actual cross section of the passage as it appears to the flow of air through it. This cross section is essentially an oval species of the racetrack type of curve referred to above.

Referring again to FIG. 3, portions of the boundary of passage 22 shown as semicircular arcs in that Figure would appear as arcs of half an ellipse if the entire sec- 7 tion were in the cylindrical surface centered on the axis of rotation of the impeller. Such half elliptical arcs will, of course, have their major axis in the plane of line 19.

With the passage configuration of constant width axially of the impeller at a given radius and with the width increasing with radius, it will be apparent that the outline or bounding surface for the diffusing passages will be two diverging or slightly dished surfaces.

It is also possible, although considered less feasible, to machine the passages so that each passage, if looked at in a plane making equal angles with the branch centerlines 30 and 31 would be of the basic racetrack form bounded by two parallel lines and two equal semicircular arcs; that is, appearing much as the passage appears in FIG. 3. In this case, the sections as in FIG. 4A would have the lines EF of equal length whereas a section taken as in FIG. 3 would have the lines C-D of unequal length. This does not provide the smooth wall at the diffuser entrance which the preferred configuration does.

FIG. 48 illustrates a slight modification of the racetrack cross section of the passage. This is a slight smoothing of that of FIG. 4A. In FIG. 4B, the lines EE and FF are tangent to the circular arcs EF. Thus, the upper larger radius arc EF is of slightly over 180 extent and the lower smaller radius arc E-F is of slightly less than 180 extent.

FIG. 6 illustrates successive cross sections of a passage taken in planes making equal angles with the branch centerlines 30 and 31 and finally normal to the common centerline 35. This illustrates a structure in which the passages of the basic racetrack form are bounded by semicircles of equal radius and parallel lines. As the branch centerlines converge, the cross section approaches a circle and finally becomes a circle at point 35 where these lines merge. It will be noted that these are illustrated to illustrate a gradual divergence of the basic passage area downstream, the radii of the semicircles increasing progressively through views G, H, J, and K. The arcs 38 and 42 in views G and H illustrate the overlap of the next adjacent downstream passage.

Any of the passage cross sections described may be machined by a suitably programmed numerically controlled machine.

Referring also now to FIG. 2, it will be appreciated that, as the branch centerline 31 of a passage diverges from the branch centerline 30 of the next passage, the overlap of adjacent passages decreases. At a point indicated as 39 on FIG. 2, the lines 31 and 30 have diverged an amount equal to the sum of the radii of the circular arcs which defines the boundaries of the passages, and therefore the separation of the passages become complete. From this point downstream in the passages they are completely separated by the dividing wall 23. Point 39 may thus be considered as the farthest downstream point of a generally elliptical leading edge 40 of the wall 23. This leading edge is the shape which is defined by the intersection of the passages.

Depending upon the curvature of lines 31 and 30 and the relation of the radii of the bounding semicircles, the leading edge 40 is most likely to depart somewhat from a true plane as indicated by the slight curvature of line 40 on FIG. 2. However, the curve is an approximation to the ellipse and will partake of any advantages that may result from the elliptical leading edge taught by the prior art referred to above. It will be appreciated that this leading edge 40 vanishes into the bounding walls of the diffuser abreast of the point 36. The wall from this point inward is smooth.

As stated above, it may be desired to have the diffusing passages converge in the initial portion to a throat or to diverge continuously or monotonically from the entrance. The initial portion of the passages may diverge simply by the disappearance of the overlap between adjoining passages 22 as they approach the point 39. This is illustrated for example by the arc 38 or 42 in FIGS. 3, 4, and 6.

Also, the area of the passage may decrease to some desired point to provide a throat in the passage, if the entering flow is supersonic and a supersonic diffuser is required up to such a throat. The reasons for providing an initial converging passage with a throat as opposed to a continuously or monotonically diverging passage are pointed out in Bandukwalla US. Pat. No. 3,778,186 issued Dec. 11, 1973, of common ownership. Such a converging portion of the passage may be simply due to a decrease in depth of the passages at constant width, or may be the result of decrease in width, or decrease in both, up to the throat.

FIG. 15 illustrates a second embodiment of the invention in which the diffusing passages are disposed so as to terminate substantially perpendicular to the outer surface 26 of the diffuser. For simplicity of explanation,

parts, lines, axes, and surfaces are identified. by.the same numerals in FIG. v as in FIG. 2. However, as will be pointed out, the form of some of these differ from those previously described. The overall outlineof the diffuser and the rotor and the housingin general may remain as previously described. The difference lies only in the diffusing passages 22. In this case the leading centerline branch is curved forward and, specifically, the trailing centerline branch 31 is straight. The

common centerline extending outwardly from the junction 34 of the branch centerline. is also curved. Specifically, in the form illustrated, the line 31 is tangent to lines 30 and 3 5 at'their point of intersection and effectively. lines 30 and 35 are a continuous curved line. Specifically, as depicted, this line is an arc of a circle which at its inception is substantially parallel to themriphery of the rotor and at its terminus nearly. perpendicular to the outer surface 26 of the diffuser. The line 31 in effect is drawn from the point 36 in which the next downstream centerline terminates to a point of tangency with the circular arc 3035.Thus, in this case, thev downstream branch centerline is straight and the upstream one is curved forwardly. The result, so far as the cross section of the passages is much the same as in the previous example. The adjacent passages intersect along the leading edge 39 of the dividing wall 23 and the passages separate at the point 39 as before. The general shape of the leading edge 40 remains essentially as previously described. 1

It will be apparent from the foregoing that a great many shapes of centerlines of the principal part of the diffuser passage and different curvatures of the branch centerlines may be adopted to suit particular conditions of desired angle of entry and dischargev of the compressed air or gas. 2

As in the previously described case, the passage may be continuously diverging or may be convergentdivergent, and any suitable variation in width of the passage and corresponding radius of the bounding circular arcs may be adopted for the desired result.

It will be apparent that the detail design for a particular installation of a compressor embodying the abovedescribed principle of branching centerlines and racetrack configuration of the basic cross section of the diffusing passage inlet may involve the skill of the compressor aerodynamicist. The aerodynamics of design of compressor diffusers has been widely explored and practiced by many persons skilled in the art. The application of such techniques tothe detailed proportioning of a particular diffuser to suit a particular rotor and particular rotor speed and inlet and discharge condi-, tions need not be specifically described, as such is part of the stock in trade of those skilled in;the design of aerodynamic machinery. Such details include overall dimensions, passage area ratio, number of passages, total flow area, and the like.

It will be apparent to those skilled in' the art that the principles described herein promote a great deal of flexibility in the design of a compressor diffuser in cases where it is desired to employ the elliptical leadingedges or a pipe diffuser.

The detailed description of preferred embodimen of the invention for the purpose of explaining the principles thereof is not to be considered as restricting or limiting the invention, since many modifications may be made by the exercise of skill in the art without de-. parting from the scope of the invention.

I claim;

, 1. A. centrifugal compressor comprising, in combination, a rotor operable to discharge gas at high velocity from the periphery of the rotor with radial and tangential components of velocity and a stator enclosing the rotor, the stator including a diffuser disposed around theperipheryof the rotorto receive and diffuse the gas discharged from the rotor, the-diffuser defining an annular array ofdiffusing passages extending radially outwardly and circumferantially from the rotor, at least the downstream portion of each passage being divergent in area in the direction of flow, the passages intersecting at the entrances to the passages and being discrete at the outlets from the passages, in which the improvement comprises passages based upon a branched centerline, each passage having a single centerline through a downstream portion of the passage, the branchesof the centerline diverging toward the entrances to each passage with one branch forward of the other with respect to the tangential component of velocity of the gas, the foward branch of each passage intersecting the other branch of the next adjacent passage at the inner periphery of the diffuser, each passage having a basic racetrack cross-section substantially as defined by opposed substantially semicircular arcs centered on the respective branches of the centerline and straight line segments connecting the arcs forward of the point of merging of the branches, adjacent basic cross-sections overlapping at the entrances to the passages so that generally elliptical leading edges of the walls separating the passages are provided at the zone of transition from intersecting to discrete passages.

2. A centrifugal compressor comprising, in combination, a rotor operable to discharge gas at high velocity from the periphery of the rotor with radial and tangential components of velocity and a stator enclosing the rotor, the stator including a diffuser disposed around the periphery of the rotor to receive and diffuse the gas discharged from the rotor, the diffuser defining an annular array of diffusing passages extending radially outwardly and circumferentially from the rotor, at least the downstream portion of each passage being divergent in area in the direction of flow, the passages intersecting at the entrances to the passages and being discrete at the outlets from the passages, in which the improvement comprises passages based upon a branched centerline, each passage having a single centerline through a downstream portion of the passage, the branches of the centerline diverging toward the entrances to each passage with one branch forward of the other with respect to the tangential component of velocity ofthe gas, the forward branch of each passage intersecting the other branch of the next adjacent passage atthe inner periphery of the diffuser, each passage having-a basic racetrack cross-section substantially as defined by opposed substantially semicircular arcs centered on the respective'branches of the centerline and straight line segments connecting the arcs forward of the point of merging of the branches, adjacent basic cross-sections overlapping at the entrances to the passages so that generally elliptical leading edges of the walls separating the passages are provided at the zone of transition from intersecting to discrete passages, the forward and common centerlines approximating a continuous straight line.

3. A centrifugal compressor comprising, in combination, a rotor operable to discharge gas at high velocity from the periphery of the rotor with radial and tangential components of velocity and a stator enclosing the rotor, the stator including a diffuser disposed around the periphery of the rotor to receive and diffuse the gas discharged from the rotor, the diffuser defining an annular array of diffusing passages extending radially outwardly and circumferentially from the rotor, at least the downstream portion of each passage being divergent in area in the direction of flow, the passages intersecting at the entrances to the passages and being discrete at the outlets from the passages, in which the improvement comprises passages based upon a branched centerline, each passage having a single centerline through a downstream portion of the passage, the branches of the centerline diverging toward the entrances to each passage with one branch forward of the other with respect to the tangential component of velocity of the gas, the forward branch of each passage intersecting the other branch of the next adjacent passage at the inner periphery of the diffuser, each passage having a basic racetrack cross-section substantially as defined by opposed substantially semicircular arcs centered on the respective branches of the centerline and straight line segments connecting the arcs forward of the point of merging of the branches, adjacent basic cross-sections overlapping at the entrances to the passages so that generally elliptical leading edges of the walls separating the passages are provided at the zone of transition from intersecting to discrete passages, the forward and common centerlines following a smooth curve which is convex as viewed from the rotor.

4. A centrifugal compressor comprising, in combination, a rotor operable to discharge gas at high velocity from the periphery of the rotor with radial and tangential components of velocity and a stator enclosing the rotor, the stator including a diffuser disposed around the periphery of the rotor to receive and diffuse the gas discharged from the rotor, the diffuser defining an annular array of diffusing passages extending radially outwardly and circumferentially from the rotor, at least the downstream portion of each passage being divergent in area in the direction of flow, the passages intersecting at the entrances to the passages and being discrete at the outlets from the passages, in which the improvement comprises passages based upon a branched centerline, each passage having a single centerline through a downstream portion of the passage, the branches of the centerline diverging toward the entrances to each passage with one branch forward of the other with respect to the tangential component of velocity of the gas, the forward branch of each passage intersecting the other branch of the next adjacent passage at the inner periphery of the diffuser, each passage having a basic racetrack cross-section substantially as defined by opposed substantially semicircular arcs centered on the respective branches of the centerline and straight line segments connecting the arcs forward of the point of merging of the branches, adjacent basic cross-sections over-lapping at the entrances to the passages so that generally elliptical leading edges of the walls separating the passages are provided at the zone of transition from intersecting to discrete passages, the straight line segments of any passage boundary being parallel in section equidistant from the axis of rotation of the rotor.

5. A centrifugal compressor comprising, in combination, a rotor operable to discharge gas at high velocity from the periphery of the rotor with radial and tangential components of velocity and a stator enclosing the rotor, the stator including a diffuser disposed around the periphery of the rotor to receive and diffuse the gas discharged from the rotor, the diffuser defining an annular array of diffusing passages extending radially outwardly and circumferentially from the rotor, at least the downstream portion of each passage being divergent in area in the direction of flow, the passages intersecting at the entrances to the passages and being discrete at the outlets from the passages, in which the improvement comprises passages based upon a branched centerline, each passage having a single centerline through a downstream portion of the passage, the branches of the centerline diverging toward the entrances to each passage with one branch forward of the other with respect to the tangential component of velocity of the gas, the forward branch of each passage intersecting the other branch of the next adjacent passage at the inner periphery of the diffuser, each passage having a basic racetrack cross-section substantially as defined by opposed substantially semicircular arcs centered on the respective branches of the centerline and straight line segments connecting the arcs forward of the point of merging of the branches, adjacent basic cross-sections overlapping at the entrances to the passages so that generally elliptical leading edges of the walls separating the passages are provided at the zone of transition from intersecting to discrete passages, the straight line segments of any passage boundary being parallel in section transverse to the direction of flow through the passage. 

1. A centrifugal compressor comprising, in combination, a rotor operable to discharge gas at high velocity from the periphery of the rotor with radial and tangential components of velocity and a stator enclosing the rotor, the stator including a diffuser disposed around the periphery of the rotor to receive and diffuse the gas discharged from the rotor, the diffuser defining an annular array of diffusing passages extending radially outwardly and circumferantially from the rotor, at least the downstream portion of each passage being divergent in area in the direction of flow, the passages intersecting at the entrances to the passages and being discrete at the outlets from the passages, in which the improvement comprises passages based upon a branched centerline, each passage having a single centerline through a downstream portion of the passage, the branches of the centerline diverging toward the entrances to each passage with one branch forward of the other with respect to the tangential component of velocity of the gas, the foward branch of each passage intersecting the other branch of the next adjacent passage at the inner periphery of the diffuser, each passage having a basic racetrack cross-section substantially as defined by opposed substantially semicircular arcs centered on the respective branches of the centerline and straight line segments connecting the arcs forward of the point of merging of the branches, adjacent basic cross-sections overlapping at the entrances to the passages so that generally elliptical leading edges of the walls separating the passages are provided at the zone of transition from intersecting to discrete passages.
 2. A centrifugal compressor comprising, in combination, a rotor operable to discharge gas at high velocity from the periphery of the rotor with radial and tangential components of velocity and a stator enclosing the rotor, the stator including a diffuser disposed around the periphery of the rotor to receive and diffuse the gas discharged from the rotor, the diffuser defining an annular array of diffusing passages extending radially outwardly and circumferentially from the rotor, at least the downstream portion of each passage being divergent in area in the direction of flow, the passages intersecting at the entrances to the passages and being discrete at the outlets from the passages, in which the improvement comprises passages based upon a branched centerline, each passage having a single centerline through a downstream portion of the passage, the branches of the centerline diverging toward the entrances to each passage with one branch forward of the other with respect to the tangential component of velocity of the gas, the forward branch of each passage intersecting the other branch of the next adjacent passage at the inner periphery of the diffuser, each passage having a basic racetrack cross-section substantially as defined by opposed substantially semicircular arcs centered on the respective branches of the centerline and straight line segments connecting the arcs forward of the point of merging of the branches, adjacent basic cross-sections overlapping at the entrances to the passages so that generally elliptical leading edges of the walls separating the passages are provided at the zone of transition from intersecting to discrete passages, the forward and common centerlines approximating a continuous straight line.
 3. A centrifugal compressor comprising, in combination, a rotor operable to discharge gas at high velocity from the periphery of the rotor with radial and tangential components of velocity and a stator enclosing the rotor, the stator including a diffuser disposed around the periphery of the rotor to receive and diffuse the gas discharged from the rotor, the diffuser defining an annular array of diffusing passages extending radially outwardly and circumferentially from the rotor, at least the downstream portion of each passage being divergent in area in the direction of flow, the passages intersecting at the entrances to the passages and being discrete at the outlets from the passages, in which the improvement comprises passages based upon a branched centerline, each passage having a single centerline through a downstream portion of the passage, the branches of the centerline diverging toward the entrances to each passage with one branch forward of the other with respect to the tangential component of velocity of the gas, the forward branch of each passage intersecting the other branch of the next adjacent passage at the inner periphery of the diffuser, each passage having a basic racetrack cross-section substantially as defined by opposed substantially semicircular arcs centered on the respective branches of the centerline and straight line segments connecting the arcs forward of the point of merging of the branches, adjacent basic cross-sections overlapping at the entrances to the passages so that generally elliptical leading edges of the walls separating the passages are provided at the zone of transition from intersecting to discrete passages, the forward and common centerlines following a smooth curve which is convex as viewed from the rotor.
 4. A centrifugal compressor comprising, in combination, a rotor operable to discharge gas at high velocity from the periphery of the rotor with radial and tangential components of velocity and a stator enclosing the rotor, the stator including a diffuser disposed around the periphery of the rotor to receive and diffuse the gas discharged from the rotor, the diffuser Defining an annular array of diffusing passages extending radially outwardly and circumferentially from the rotor, at least the downstream portion of each passage being divergent in area in the direction of flow, the passages intersecting at the entrances to the passages and being discrete at the outlets from the passages, in which the improvement comprises passages based upon a branched centerline, each passage having a single centerline through a downstream portion of the passage, the branches of the centerline diverging toward the entrances to each passage with one branch forward of the other with respect to the tangential component of velocity of the gas, the forward branch of each passage intersecting the other branch of the next adjacent passage at the inner periphery of the diffuser, each passage having a basic racetrack cross-section substantially as defined by opposed substantially semicircular arcs centered on the respective branches of the centerline and straight line segments connecting the arcs forward of the point of merging of the branches, adjacent basic cross-sections over-lapping at the entrances to the passages so that generally elliptical leading edges of the walls separating the passages are provided at the zone of transition from intersecting to discrete passages, the straight line segments of any passage boundary being parallel in section equidistant from the axis of rotation of the rotor.
 5. A centrifugal compressor comprising, in combination, a rotor operable to discharge gas at high velocity from the periphery of the rotor with radial and tangential components of velocity and a stator enclosing the rotor, the stator including a diffuser disposed around the periphery of the rotor to receive and diffuse the gas discharged from the rotor, the diffuser defining an annular array of diffusing passages extending radially outwardly and circumferentially from the rotor, at least the downstream portion of each passage being divergent in area in the direction of flow, the passages intersecting at the entrances to the passages and being discrete at the outlets from the passages, in which the improvement comprises passages based upon a branched centerline, each passage having a single centerline through a downstream portion of the passage, the branches of the centerline diverging toward the entrances to each passage with one branch forward of the other with respect to the tangential component of velocity of the gas, the forward branch of each passage intersecting the other branch of the next adjacent passage at the inner periphery of the diffuser, each passage having a basic racetrack cross-section substantially as defined by opposed substantially semicircular arcs centered on the respective branches of the centerline and straight line segments connecting the arcs forward of the point of merging of the branches, adjacent basic cross-sections overlapping at the entrances to the passages so that generally elliptical leading edges of the walls separating the passages are provided at the zone of transition from intersecting to discrete passages, the straight line segments of any passage boundary being parallel in section transverse to the direction of flow through the passage. 